1. Field of the Invention
The present invention relates to an apparatus for detecting the ground-speed of a motor vehicle, namely, the running speed of the vehicle with respect to the road surface, by utilizing the Doppler effect, and more particularly to a technique for improving the accuracy of detection of the ground-speed.
2. Discussion of the Prior Art
A Doppler-effect vehicle ground-speed detecting apparatus is known according to JP-A 61-14586 (published in 1986), which includes (a) a transmitter fixed to a body of a vehicle, for transmitting a wave toward the road surface on which the vehicle is running, (b) a receiver fixed to the vehicle body, for receiving a portion of the wave which is reflected by the road surface, and (c) ground-speed determining means for determining the ground-speed of the vehicle relative to the road surface, on the basis of the transmitting frequency of the wave as transmitted from the transmitter, and the receiving frequency of the wave as received by the receiver.
The transmitter and receiver are fixed to the vehicle body such that the transmitter and receiver are oriented so as to face in a predetermined direction relative to the running direction of the vehicle, so that the paths along which the wave is transmitted from the transmitter and received by the receiver are at a predetermined angle relative to the road surface in a plane parallel to the running direction of the vehicle and perpendicular to the road surface. In other words, the angle of the wave transmission and reception paths relative to the road surface changes when the angle of the vehicle body in the above-indicated plane relative to the road surface changes from the nominal angle due to pitching or inclination of the vehicle body.
For example, the vehicle body undergoes pitching or inclination upon brake application or upon quick or abrupt acceleration or deceleration. Accordingly, the angle of the wave transmission and reception paths relative to the road surface deviates from the nominal angle. This deviation has an influence on the ground-speed which is detected by the detecting apparatus on the basis of the transmitting and receiving frequencies of the transmitter and receiver. Thus, the detected ground-speed of the vehicle more or less includes an error which arises from a variation in the angle of the transmitter and receiver relative to the road surface while the vehicle is running. For this reason, the conventional Doppler-effect vehicle ground-speed detecting apparatus is not satisfactory in terms of the detecting accuracy.